Optical scanning device

ABSTRACT

Disclosed is an optical scanning device, including a mirror, a first drive beam configured to swing the mirror around a first axis, and a second drive beam configured to swing the mirror around a second axis, wherein the second drive beam is provided in such a manner that a plurality of beams extending in a direction intersecting with a direction of the second axis are joined with adjacent beams at edge portions thereof, and thereby has a zigzag shape, and each of the plurality of beams includes a rib extending in a direction of a width of the beam.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a division and claims benefit under 35 U.S.C.120 of copending U.S. patent application Ser. No. 13/330,767, filed Dec.20, 2011, which claims the benefit of priority to Japanese PatentApplication No. 2010-286758 filed on Dec. 22, 2010, the entire contentsof which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to an optical scanning device,and more specifically to an optical scanning device that supports amirror supporting part from both sides in an axis direction by a pair oftorsion beams, and drives the mirror supporting part so as to swingaround the axis by twisting the torsion beams.

2. Description of the Related Art

Conventionally, an optical scanning device is known that includes amovable plate that reflects an incident light, a torsion beam thatsupports the movable plate in a rotatable way around an axial direction,and a drive part that gives a drive force in a twisting direction to thetorsion beam, wherein a rib is formed at least in the vicinity of aconnection between the movable plate and the torsion beam, as disclosedin Japanese Patent Application Laid-Open Publication No. 2010-128116(which is hereinafter called “Patent Document 1”).

The optical scanning device disclosed in Patent Document 1 aims atsuppressing a dynamic distortion of a reflecting plane withoutincreasing a weight of the movable plate.

In recent years, optical scanning devices tend to be required to havehigher resolution. To implement the higher resolution, a resonantfrequency is needed to be raised, for which a rigidity of a torsion beam(which may be called a “torsion bar”) is needed to be increased.

Here, if a width of the torsion beam is broadened to improve therigidity, since a deformed state of the torsion beam differs dependingon a distance from a center axis of the torsion, a problem ofnonlinearity of a displacement is caused.

In Patent Document 1, since such a problem of the nonlinear oscillationis not considered at all, if the resonant frequency is raised, theproblem of the nonlinearity of the displacement is caused. Moreover, inthe configuration described in Patent Document 1, an effect ofpreventing a mirror deformation is obtained, but a stress caused by theoscillation is not blocked, so if the resonant frequency is raised, themirror deformation is not prevented.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide a novel and useful opticalscanning device solving one or more of the problems discussed above.

More specifically, embodiments of the present invention provide anoptical scanning device to be able to reduce an occurrence of anonlinear oscillation and a generated stress, and to be able to preventa deformation of a mirror even if driven at a high resonant frequency.

According to one aspect of the present invention, an optical scanningdevice is provided, the device including:

-   -   a mirror;    -   a mirror supporting part to support the mirror on an upper        surface; and    -   a pair of torsion beams to support the mirror supporting part        from both sides in an axis direction and to drive the mirror        supporting part so as to swing the mirror supporting part around        the axis by being twisted themselves,    -   wherein each of the torsion beams includes a slit approximately        parallel to the axis direction.

Additional objects and advantages of the embodiments are set forth inpart in the description which follows, and in part will become obviousfrom the description, or may be learned by practice of the invention.The objects and advantages of the invention will be realized andattained by means of the elements and combinations particularly pointedout in the appended claims. It is to be understood that both theforegoing general description and the following detailed description areexemplary and explanatory and are not restrictive of the invention asclaimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a top perspective view showing an example of an opticalscanning device of a first embodiment of the present invention;

FIG. 1B is a bottom perspective view showing an example of the opticalscanning device of the first embodiment of the present invention;

FIG. 2A is a enlarged view showing a part A of FIG. 1A;

FIG. 2B is a enlarged view showing a part B of FIG. 1A;

FIG. 3A is a view showing a configuration inside a movable frame of acomparative example;

FIG. 3B is a perspective view showing an enlarged torsion beam of theoptical scanning device of the comparative example;

FIG. 3C is a cross-sectional view of the torsion beam of the opticalscanning device of the comparative example;

FIG. 3D is a view showing a beam with a square cross section;

FIG. 4A is a view showing an example of a frequency/displacementcharacteristic of a linear oscillation;

FIG. 4B is a view showing an example of a frequency/displacementcharacteristic of a nonlinear oscillation;

FIG. 4C is a view showing an example of a frequency/displacementcharacteristic when the nonlinear oscillation strongly appears;

FIG. 5A is a view showing a configuration inside a movable frame of theoptical scanning device of the first embodiment;

FIG. 5B is an enlarged view of the torsion beam of the optical scanningdevice of the first embodiment;

FIG. 5C is a view showing a cross-sectional configuration of the torsionbeam of the optical scanning device of the first embodiment;

FIG. 6A is a view showing performance results of an optical scanningdevice of a comparative example without a slit;

FIG. 6B is a view showing performance results of an optical scanningdevice of a first example including a slit;

FIG. 7A is a view showing a displacement/frequency characteristic of theoptical scanning device of the first embodiment;

FIG. 7B is a view showing a displacement/frequency characteristic when asquareness ratio of a torsion beam divided by a slit is changed;

FIG. 7C is a view showing a displacement/frequency characteristic of anoptical scanning device of a comparative example;

FIG. 8A is an enlarged view showing the upper side of a torsion beamwhen a short slit is provided in the torsion beam;

FIG. 8B is an enlarged view showing the back side of the torsion beamwhen the short slit is provided in the torsion beam;

FIG. 8C is a stress distribution map showing the back side of thetorsion beam when the short slit is provided in the torsion beam;

FIG. 9A is a configuration view showing the upper side of a connectionbetween a mirror supporting part and a torsion beam of an opticalscanning device of the first embodiment;

FIG. 9B is a configuration view showing the back side of the connectionbetween the mirror supporting part and the torsion beam of the opticalscanning device of the first embodiment;

FIG. 10 is a map showing a stress distribution at an edge of the slit ofthe torsion beam in the optical scanning device of the first embodiment;

FIG. 11A is a view showing an example of a deformation distribution of amirror of an optical scanning device having a configuration without amirror deformation prevention structure;

FIG. 11B is a view showing an example of a stress distribution of themirror of the optical scanning device having the configuration withoutthe mirror deformation prevention structure;

FIG. 12 is a view to illustrate a mirror deformation preventionstructure of the optical scanning device of the first embodiment;

FIG. 13A is a perspective view showing a rib structure of the mirrorsupporting part of the optical scanning device of the first embodiment;

FIG. 13B is a plan view showing the rib structure of the mirrorsupporting part of the optical scanning device of the first embodiment;

FIG. 14A is a view showing a mirror deformation amount of the opticalscanning device of the first embodiment;

FIG. 14B is a view showing a stress distribution of the optical scanningdevice of the first embodiment including a projecting part of aconnecting rib;

FIG. 14C is a view showing a stress distribution in a mirror plane;

FIG. 15A is a cross-sectional configuration view of a torsion beam of anoptical scanning device of a second embodiment;

FIG. 15B is a plane configuration view of the back side of the opticalscanning device of the second embodiment;

FIGS. 15C and 15D are views showing the relationship among a projectingamount X of a connecting rib, a mirror flatness λ in a maximuminclination and a nonlinear coefficient μ;

FIG. 16A is a perspective view showing a configuration on the upper sideof an optical scanning device using a movable frame and not including arib on the back side;

FIG. 16B is a view showing a configuration on the back side of theoptical scanning device using the movable frame and not including therib on the back side;

FIG. 16C is a view showing a horizontal driving state of the opticalscanning device using the movable frame and not including the rib on theback side;

FIG. 17A is a perspective view showing a configuration on the upper sideof an optical scanning device using a movable frame and including a ribon the back side;

FIG. 17B is a view showing a configuration on the back side of theoptical scanning device using the movable frame and including the rib onthe back side;

FIG. 17C is a view showing a horizontal driving state of the opticalscanning device using the movable frame and including the rib on theback side;

FIG. 18A is a perspective view showing a configuration on the upper sideof the optical scanning device of the first embodiment;

FIG. 18B is a perspective view showing a configuration on the back sideof the optical scanning device of the first embodiment;

FIG. 18C is an enlarged view showing a crosstalk prevention structure ofthe optical scanning device of the first embodiment;

FIG. 19 is a stress distribution map in horizontal driving of theoptical scanning device of the first embodiment;

FIG. 20A is a plan configuration view showing an optical scanning devicewithout a frequency change prevention structure;

FIG. 20B is a cross-sectional configuration view of a movable frame anda resonant drive beam of the optical scanning device without thefrequency change prevention structure shown in FIG. 20A;

FIG. 20C is a view showing a drive state of the resonant drive beam;

FIG. 20D is a view showing the relationship between an integrated drivetime of the resonant drive beam and a resonant frequency change rate;

FIG. 21A is a stress distribution map on the upper side in a horizontaldriving of an optical scanning device without a frequency changeprevention structure;

FIG. 21B is a stress distribution map on the back side in the horizontaldriving of the optical scanning device without the frequency changeprevention structure;

FIG. 21C is an enlarged view showing a supporting part of a drive beamof a stress distribution on the back side in the horizontal driving ofthe optical scanning device without the frequency change preventionstructure;

FIG. 22A is a plane configuration view of an optical scanning devicewith a first frequency change prevention structure;

FIG. 22B is an enlarged view showing a root part shown in FIG. 22A;

FIG. 23A is a stress distribution map on the upper side of a first drivebeam of an optical scanning device of a first embodiment including afirst frequency change prevention structure in a horizontal driving;

FIG. 23B is a stress distribution map on the back side of the firstdrive beam of the optical scanning device of the first embodimentincluding the first frequency change prevention structure in ahorizontal driving;

FIG. 23C is an enlarged view of a root part of the first drive beamshown in FIG. 23B;

FIG. 24 is a view showing the relationship between an integrated drivetime and a resonant frequency change rate caused by a resonant drive ofthe optical scanning device of the first embodiment;

FIG. 25A is a plan view showing the upper side of an optical scanningdevice of a first embodiment including first and second frequency changeprevention structures;

FIG. 25B is a plan view showing the back side of the optical scanningdevice of the first embodiment including the first and second frequencychange prevention structures;

FIG. 25C is an enlarged plan view showing an inside of a movable frameof the optical scanning device of the first embodiment;

FIG. 25D is an enlarged view showing a stress distribution of a lateraledge of the first drive beam of the optical scanning device of the firstembodiment;

FIG. 26A is a stress distribution map of an optical scanning devicewithout a frequency change prevention structure;

FIG. 26B is a stress distribution map of an optical scanning device withonly a first frequency change prevention structure;

FIG. 26C is a stress distribution map of an optical scanning device of afirst embodiment with first and second frequency change preventionstructures;

FIG. 27 is a view showing stress measurement results in a root of thefirst drive beam with respect to each frequency change preventionstructure shown in FIGS. 26A through 26C;

FIG. 28 is a view showing an example of an optical scanning device of asecond embodiment;

FIG. 29 is a view showing an example of an optical scanning device of athird embodiment;

FIG. 30 is a view showing an example of an optical scanning device of afourth embodiment; and

FIG. 31 is a view showing an example of an optical scanning device of afifth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description is given below, with reference to drawings of embodimentsof the present invention.

First Embodiment

(Overall Structure)

FIGS. 1A and 1B are perspective views showing an example of aconfiguration of an optical scanning device of a first embodiment of thepresent invention. FIG. 1A is a top perspective view showing an exampleof the optical scanning device of the first embodiment. FIG. 1B is abottom perspective view showing an example of the optical scanningdevice of the first embodiment.

In FIGS. 1A and 1B, the optical scanning device of the present inventionincludes a mirror 10, a mirror supporting part 20, torsion beams 30,coupling beams 40, first drive beams 50, a movable frame 60, seconddrive beams 70, crosstalk preventing ribs 80, and a fixed frame 90. Thetorsion beams 30 include a slit 31. Moreover, as shown in FIG. 1A, thefirst drive beams 50 include a drive source 51, and the second drivebeams 70 include a drive source 71. Furthermore, as shown in FIG. 1B, arib is provided on the back side of the mirror supporting part 20, andharmonic superposition preventing ribs 72 are provided on the back sideof the second drive beams 70.

In FIGS. 1A and 1B, the mirror 10 is supported on the upper surface ofthe mirror supporting part 20, and the mirror supporting part 20 isconnected to edges of the torsion beams 30 located on both sides of themirror supporting part 20. The torsion beams 30 form a rocking axis,extend in an axial direction, and support the mirror supporting part 20from both sides in the axial direction. By twisting the torsion beams30, the mirror 10 supported by the mirror supporting part 20 swings andperforms an operation of deflecting a reflected light of an incidentlight on the mirror 10. The torsion beams 30 are coupled and supportedby the coupling beams 40, and connected to the first drive beams 50. Thefirst drive beams 50, the coupling beams 40, the torsion beams 30, themirror supporting part 20 and the mirror 10 are surrounded by themovable frame 60. The first drive beams 50 are supported by the movableframe 60 at one side, and extend inward so as to be connected to thecoupling beams 40. Two first drive beams 50 are provided so as tosandwich the mirror 10 and the mirror supporting part 20 in a directionperpendicular to the torsion beams 30. A thin film of a piezoelectricdevice is formed on the upper surface of the first drive beam 50 as thedrive source 51. Since the piezoelectric device expands and contractsdepending on the polarity of an applied voltage, by alternately applyingdifferent voltages in phase to the left first drive beam 50 and theright first drive beam 50, the first drive beams 50 on the left andright of the mirror alternately oscillate up and down oppositely, bywhich the mirror 10 can be swung around the axis, making the torsionbeams 30 serve as a rocking axis or a rotation axis. The direction wherethe mirror 10 swings around the torsion beams 30 is hereinafter called a“horizontal direction”. Generally speaking, the horizontal directioncorresponds to a horizontal direction of a screen for projection. In thescreen, a lateral direction is generally called the horizontaldirection. For example, resonant drive may be used for a horizontaldrive by the first drive beams 50, and the mirror 10 may be driven andswung at high speed. Moreover, one edge of each of the second drivebeams 70 is coupled to the outside of the movable frame 60. By applyingdifferent polarities of voltages to the adjacent drive sources 71 perrectangle unit, the adjacent rectangular beams are recurved invertically opposite directions, and an integration of an up-and-downmotion of the rectangular beams can be transmitted to the movable frame60. Then the mirror 10 can be swung in a direction perpendicular to thehorizontal direction, in a vertical direction. As mentioned above, thevertical direction generally corresponds to a vertical direction of ascreen for projection, a longitudinal direction of the screen. Forexample, the second drive beams 70 may generate the drive force bynon-resonant oscillation.

The optical scanning device of the present embodiment may be implementedby various materials and processing methods as long as the opticalscanning device has the above mentioned configuration and theconfiguration is practicable. For example, the optical scanning deviceof the present embodiment may be implemented by Micro Electro MachineSystem (MEMS) technology by using semiconductor fabrication. Forexample, if a Silicon On Insulator (SOI) substrate is used, byprocessing the substrate so as to leave only a silicon substrate on theupper side as thin beam parts, and by processing the substrate so as toalso leave a silicon substrate on the back side as thick frames andribs, a structural body of the optical scanning device can be producedreadily.

The optical scanning device of the present embodiment can be configuredas a piezoelectric dual-axis drive type actuator mountable on a microprojector, and can be inexpensively produced to have a small size and ahigh performance. Here, for example, the “small size” means a height notmore than 7 mm, and the “high performance” means that a high-speeddrawing can be performed on an A3 size screen at a 50 cm distance at anXGA resolution (i.e., eXtended Graphic Array, a resolution of 1024*768pixels) or 720 p. The optical scanning device of the present embodimentmay be, for example, configured as a small-size and high-performancedual axis drive type micro mirror actuator including a non-resonantdrive type actuator mechanism that swings in a vertical direction at amechanical angle ±9 degrees at 60 Hz in a saw-tooth pattern and aresonant drive type actuator mechanism that swings in a horizontaldirection at a mechanical angle ±12 degrees at a resonant frequency 25kHz.

FIGS. 2A and 2B are enlarged views showing A part and B part of FIG. 1A.FIG. 2A is an enlarged view showing the A part of FIG. 1A. FIG. 2B is anenlarged view showing the B part of FIG. 1A.

In FIG. 2A, four of mirror horizontal angle sensors 100 are provided onthe coupling beams 40. The mirror horizontal angle sensors 100 aresensors that detect an inclination angle of the mirror 10 in ahorizontal direction. Since the coupling beams 40 reflect theinclination angle of the mirror 10 in the horizontal direction, byproviding the mirror horizontal angle sensors 100 on the coupling beams40, the inclination angle of the mirror 10 of the horizontal directioncan be detected. The mirror horizontal angle sensors 100 may be, forexample, configured to have a thin film of a piezoelectric device, todetect a voltage excited in the thin film of the piezoelectric deviceaccording to the inclination angle, and to detect the inclination angleof the horizontal direction.

In FIG. 2B, a mirror vertical angle sensor 101 is provided on the seconddrive beam 70. Since a drive in a vertical direction is reflected in amotion of the second drive beams 70, for example, the mirror verticalangle sensor 101 may be provided on the second beam 70. For example, themirror vertical angle sensor 101 may be also configured to use apiezoelectric device as mentioned above.

Next, with respect to details of the optical scanning device of thepresent embodiment, descriptions are given for respective componentparts sequentially. Here, in the optical scanning device of the firstembodiment, if there is a practical example such as measurement results,the example is taken and referred to for each component part.

(Slit Structure)

FIGS. 3A through 3D are views showing a comparative example toillustrate a slit structure of the optical scanning device of thepresent embodiment. FIGS. 3A through 3D show an optical scanning deviceof the comparative example including torsion beams 130 in which a slit31 is not formed, different from the present embodiment. Here, in FIGS.3A through 3D, numerals similar to FIGS. 1 and 2 are put to componentssimilar to ones of the optical scanning device of the presentembodiment, and different numerals are put to components different fromthe optical scanning device of the present embodiment.

FIG. 3A is a view showing a configuration within a movable frame 60 ofthe optical scanning device of the comparative example. As shown in FIG.3A, the optical scanning device of the comparative example differs fromthe optical scanning device of the present embodiment in that the slits31 are not formed in torsion beams 130, inside the movable frame 60.

FIG. 3B is an enlarged perspective view showing the torsion beams 130 ofthe optical scanning device of the comparative example. As shown in FIG.3B, the torsion beams 130 have a lamellar shape with a greater widthcompared to a thickness. Since high resolution is required in recentyears, scanning many pixels per unit time is needed, and speeding up theswinging drive for scanning is needed. To implement speeding up, theresonant frequency to drive the mirror 10 has to be raised. To do this,rigidity of the torsion beams 130 have to be increased. This is becauseif the optical scanning device is configured by using a semiconductorprocess with MEMS technology, since the thickness of the thin parts isdetermined by the rigidity related to swinging sensitivity of the seconddrive beams 70 and a primary resonant frequency fo, and all of the thinparts are configured to be constant, the horizontal width of the torsionbeams 130 needs to be increased in order to raise the rigidity.

FIG. 3C is a cross-sectional view of the torsion beam 130 of the opticalscanning device of the comparative example. As shown in FIG. 3C, thecross-section of the torsion beam 130 has a rectangle shape with agreater width compared to the thickness. This is because, as mentionedabove, the beam width of the torsion beam 130 is broadened and therigidity is improved. Then, when the torsion beam 130 is twisted, acenter part Ct, an edge Eg and a middle part Md deform differentlydepending on the position. If the mirror angle is largely changed bytwisting, the differences of the deformation state among the positionsCt, Md and Eg appear as a nonlinear displacement.

FIGS. 4A through 4C are views showing frequency/displacementcharacteristics of a linear resonant oscillation and a nonlinearresonant oscillation. FIG. 4A is a view showing an example of afrequency/displacement characteristic of a linear resonant oscillation.As shown in FIG. 4A, the linear resonant oscillation performs asymmetric oscillation, making a resonant frequency fa the center.

FIG. 4B is a view showing a frequency/displacement characteristic of anonlinear oscillation. As shown in FIG. 4B, in the nonlinear resonantoscillation, a balance of the right and left breaks, and a mountain ofthe resonant frequency leans to the right or left. In FIG. 4B, themountain of the resonant frequency leans toward the right.

FIG. 4C is a view showing examples of frequency/displacement if thenonlinear resonant oscillation intensely appears. As shown in FIG. 4C,the biggest problem when the nonlinear resonant oscillation intenselyappears is that a displacement at the drive frequency f does not changeeven if the drive voltage is changed in a range from V1 to V3, makingthe drive frequency f constant. In other words, because the peak isinclined, the displacement is increased or decreased in the inclineddirection even though the drive voltage is increased or decreased, sothere is a phenomenon where the displacement is not increased anddecreased at all at a point of the frequency f. This prevents aprojection size of a laser light from being changed freely by adjustingan applied voltage.

Next, the description is given, with reference to FIG. 3D. FIG. 3D is aview showing a beam with a square cross section. In FIG. 3D, the widthis shown by W, and the thickness is shown by T. The most efficientmeasures to prevent the nonlinearity are to change the cross-sectionalshape of the beam from the rectangle shown in FIG. 3C to the squareshown in FIG. 3D.

However, in order to make the cross-sectional shape a square whilekeeping a drive frequency constant, the thickness T needs to beincreased from the rectangular shape in FIG. 3C. However, if thethickness T is simply increased, the thickness of the second drive beams70 that are the vertical non-resonant drive structure is also increased,and desired vertical drive voltage sensitivity cannot be obtained.

Therefore, in the optical scanning device of the present embodiment, theslit 31 is provided in the torsion beam 30; the cross sections on bothsides of the slit 31 are respectively made a square; the width of thetorsion beam 30 is broadened as a whole; and the torsion beam 30 isconfigured to maintain the rigidity.

FIGS. 5A through 5C are views to explain the torsion beams 30 of theoptical scanning device of the first embodiment. FIG. 5A is a viewshowing an inside configuration of the movable frame 60 of the opticalscanning device of the first embodiment. In FIG. 5A, each of the torsionbeams 30 of the present embodiment includes the slit 31.

FIG. 5B is an enlarged view showing the torsion beam 30 of the opticalscanning device of the first embodiment. As shown in FIG. 5B, thetorsion beam 30 of the optical scanning device of the present embodimentincludes the slit 31 parallel to the axial direction. In FIG. 5B, sinceonly a single slit 31 is provided in the center of the torsion beam 30,the slit 31 is provided in a position corresponding to the rocking axis.Moreover, the slit 31 does not reach the inside edge or the outside edgeof the torsion beam 30, and is not configured to divide the torsion beam30.

FIG. 5C is a view showing an example of a cross-sectional configurationof the torsion beam 30 of the optical scanning device of the firstembodiment. As shown in FIG. 5C, by forming the slit 31 in the center ofthe torsion beam 30, the torsion beam 30 is divided into the lefttorsion beam 30L and the right torsion beam 30R in the cross sectionincluding the slit 31. Both the left torsion beam 30L and the righttorsion beam 30R have a cross section similar to a square. Accordingly,a rotation center 30LC of the left torsion beam 30L and a rotationcenter 30RC of the right torsion beam 30R both become the center ofrespective torsion beams 30L, 30R, and a difference by a displacementdoes not occur, by which the nonlinear oscillation can be reduced.Furthermore, the torsion beam 30 is assumed to swing around an assumedrotation center 31C as a whole, making it possible for the mirror 10 toswing in a horizontal direction in a desired way.

In this manner, according to the optical scanning device of the presentembodiment, by providing the slit 31 parallel to the axial direction inthe torsion beam 30, and by making the respective divided torsion beams30L, 30R have a shape similar to a square in a cross section includingthe slit 31, generation of nonlinear oscillation is suppressed.

First Example

FIGS. 6A and 6B are views showing performance results of an opticalscanning device of a first example. FIG. 6A is a view showingperformance results of an optical scanning device of a comparativeexample without the slit 31. FIG. 6B is a view showing performanceresults of an optical scanning device of the first example with the slit31.

FIG. 6A shows moments in a case where the optical scanning device of thecomparative example is displaced linearly and nonlinearly. FIG. 6A showsif the moments between the linearity and the nonlinearity disagree, thenonlinearity is intense, and if the moments between the linearity andthe nonlinearity agree, the nonlinearity does not occur. In FIG. 6A, themoments of the linearity and the nonlinearity do not overlap, theoptical scanning device of the comparative example including the torsionbeams 130 without the slits 31 shows a characteristic with intensenonlinearity.

On the other hand, FIG. 6B shows moments in a case where the opticalscanning device of the first example having a configuration similar tothe first embodiment is displaced linearly and nonlinearly. In FIG. 6B,the moments of the linearity and the nonlinearity overlap with eachother, which shows that nonlinearity does not occur.

FIGS. 7A through 7C are views showing a displacement/frequencycharacteristic of the optical scanning device of the first example andthe comparative example. FIG. 7A is a view showing thedisplacement/frequency characteristic of the optical scanning device ofthe first example. FIG. 7B shows a displacement/frequency characteristicwhen a squareness ratio of the torsion beams 30L, 30R divided by theslit 31 changes. FIG. 7C is a view showing the displacement/frequencycharacteristic of the optical scanning device of the comparativeexample.

As shown in FIG. 7A, the optical scanning device of the first exampleincludes only minimal nonlinearity, and has a characteristic that canincrease or decrease displacement by frequency f, depending on increaseor decrease of a drive voltage. This makes it possible to increase ordecrease an irradiation area of light by increasing or decreasing thedrive voltage.

Here, as shown in FIG. 7B, even if the slit 31 is provided, when thesquareness ratio is changed with respect to the right torsion beam 30Rand the left torsion beam 30L of the slit 31, and a generated stress iseased up, the nonlinearity occurs. However, the nonlinearity weakens,and increasing or decreasing the displacement according to the drivevoltage when frequency f is constant becomes possible.

Therefore, FIGS. 7A and 7B prove that by providing the slit 31 in thetorsion beam 30, if the frequency is constant and the drive voltage isincreased or decreased, increasing or decreasing the displacement ispossible.

On the other hand, FIG. 7C proves that where the nonlinearity isintense, even if the frequency is made constant and the drive voltage isincreased or decreased, the displacement does not change, and the sizeof the scanning area cannot be changed.

In this way, according to the optical scanning device of the firstexample, by providing the slit 31 approximately parallel to the axialdirection in the center of the torsion beam 30, reducing thenonlinearity is possible. In this case, the slit 31 agrees with therotation axis or the rocking axis.

Here, it is also possible to provide plural slits 31 in the torsion beam30 symmetrically about the rotation axis. However, if the number of theslits 31 is increased to two, three and more symmetrically about therotation axis, the nonlinearity further weakens, but the rigidity as theaxis beam also weakens. In order to reinforce the rigidity, if many ribs21 are provided on the back side of the mirror supporting part 20, inthat case, the gravity center of the mirror moves downward from therotation axis in a thickness direction, which generates a pendulummotion. Hence, if the number of the slits 31 is increased, the number ofthe slits 31 needs to be increased considering a balance with therigidity. Here, even if the slit 31 is only a single slit, the pendulummotion itself occurs. However, because torsional rigidity of the torsionbeam 30 is strong enough, even when the mirror 10 swings at a mirrorinclination of ±12 degrees mechanical angle, a displacement amount ofthe pendulum motion is minute, and there is no problem.

(Displacement Expansion Structure by Stress Dispersion)

FIGS. 8A through 8C are views to illustrate points to be consideredwhere the slit 31 is provided in the torsion beam 30 in the opticalscanning device of the first embodiment for nonlinearity oscillationmeasures. FIG. 8A is an enlarged view of the upper side of the torsionbeam 30 including a short slit 131. FIG. 8B is an enlarged view of theback side of the torsion beam 30 including the short slit 131. FIG. 8Cis a view showing a stress distribution on the back side of the torsionbeam 30 including the short slit 131.

FIGS. 8A and 8B show a case where an edge of the slit 131 provided inthe torsion beam 30 contacts an edge face of a rib 121 provided on theback side of the mirror supporting part 20. In such a case, as shown inFIG. 8C, stress concentrates on the edge of the slit 131 and damageeasily occurs, which causes a problem of not being able to incline themirror 10 sufficiently. Such a phenomenon also occurs if the edge of theslit 131 does not reach the edge face of the rib 121.

FIGS. 9A and 9B are views showing an example of a configuration of aconnection between the mirror supporting part 20 and the torsion beam 30of the optical scanning device of the first embodiment. To prevent thegeneration of the stress concentration on the edge of the slit 31illustrated in FIGS. 8A through 8C, the optical scanning device of thefirst embodiment adopts a configuration shown in FIGS. 9A and 9B.

FIG. 9A is a view showing an example of a configuration on the upperside of the connection between the mirror supporting part 20 and thetorsion beam 30 of the optical scanning device of the first embodiment.FIG. 9B is a view showing an example of a configuration on the back sideof the connection between the mirror supporting part 20 and the torsionbeam 30 of the optical scanning device of the first embodiment.

In FIG. 9A, an edge 31E of the slit 31 provided in the torsion beam 30cuts more inward than an outer edge of the rib 21 on the back side ofthe mirror supporting part 20, a part of the rib 21 is configured to beexposed from the slit 31. Thus, by reaching the edge 31E of the slit 31more inward than the outer edge of the rib 21, the rib 21 reinforces theedge 31E of the slit 31, and can absorb and reduce the stress generatedat the edge 31E of the slit 31.

In FIG. 9A, a thin film of a black resist 32 is formed between themirror 10 and the torsion beam 30. The black resist 32 is formed toprevent light from being reflected from a space between the mirror 10and the torsion beam 30 outside the mirror 10, if the light isirradiated in a range beyond the mirror 10. For example, the blackresist 32 may be formed by application.

As shown in FIG. 9B, the slit 31 reaches inside of the rib 21 located inthe connection between the mirror supporting part 20 and the torsionbeam 30.

FIG. 10 is a view showing a stress distribution in the edge 31E of theslit 31 of the torsion beam 30 in the optical scanning device of thefirst embodiment. FIG. 10 shows that the stress generated at the edge31E of the slit 31 does not concentrate on the edge 31E but disperses inthe torsion beam 30. If FIG. 10 is compared to FIG. 8C, the differenceis made clear.

In this way, by the edge of the slit 31 on the mirror supporting part 20provided in the torsion beam 30 reaching more inward than the outsideedge of the rib 21, and by making a configuration where the slit 31 cutsinto the mirror 10 side, the stress generated at the slit edge 31E canbe dispersed into an area other than the slit edge 31E, and incliningthe mirror 10 in a large displacement becomes possible.

(Mirror Deformation Prevention Structure)

FIGS. 11A and 11B are views showing an example of mirror deformation anda stress distribution in an optical scanning device of a configurationwithout a mirror deformation prevention structure. FIG. 11A is a viewshowing an example of a deformation distribution of the mirror 10 of theoptical scanning device of the configuration without the mirrordeformation prevention structure. FIG. 11B is a view showing an exampleof a stress distribution of the mirror 10 of the optical scanning deviceof the configuration without the mirror deformation preventionstructure.

In FIG. 11A, a vertical line passing through the center of the mirror 10becomes a rocking axis. As shown in FIG. 11A, deformations in thefarthest parts from the center on a diameter perpendicular to therocking axis, and in parts symmetrical about the rocking axis betweenthe farthest parts are large.

FIG. 11B is similar to FIG. 11A in that a vertical line passing throughthe center of the mirror 10 is a rocking axis. FIG. 11B shows that partswith high stress generated in the mirror 10 are connections with thetorsion beams 30.

FIG. 12 illustrates a mirror deformation prevention structure of theoptical scanning device of the first embodiment. In FIG. 12, parts witha large deformation of the mirror 10 are shown as A-F. In the opticalscanning device of the first embodiment, by providing ribs 21 connectingsuch parts with the large mirror deformation to each other on the backside of the mirror supporting part 20, and by further providing ribs 21on the connections of the border between the torsion beam 30 and themirror supporting part 20, a maximum mirror deformation preventioneffect is obtained with a minimum number of ribs.

FIGS. 13A and 13B are views showing a rib structure on the back side ofthe mirror supporting part 20 of the optical scanning device of thefirst embodiment. FIG. 13A is a perspective view showing a rib structureof the mirror supporting part 20 of the optical scanning device of thefirst embodiment. FIG. 13B is a plan view showing a rib structure of themirror supporting part 20 of the optical scanning device of the firstembodiment.

In FIGS. 13A and 13B, the ribs 21 are provided so as to connect thepoints A-F having high stress. More specifically, the rib structureincludes the arc-like ribs 23 that respectively connect A with B, and Cwith D arcuately, chordal ribs 24 that connect the both edges of thearc-like ribs 23 to each other and reinforce the arc-like ribs 23, atransverse rib 26 that connects E with F in a direction perpendicular tothe rocking axis, and longitudinal ribs 25 that connect A with C, and Bwith D in a direction parallel to the rocking axis. With such ribs23-26, the deformation of the mirror 10 can be directly suppressed.

However, as shown in FIG. 11B, it is thought that the stress of thetorsion beam 30 comes from the connection with the mirror supportingpart 20 to the mirror 10, and acts on the mirror 10 so as to bedeformed, so measures for the stress is needed. Therefore, in theoptical scanning device of the present embodiment, connecting ribs 22are also provided on the connection between the torsion beam 30 and themirror supporting part 20. Furthermore, as shown in FIG. 13B, byprojecting the connecting rib 22 farther toward the torsion beam 30 thanis the edge of the mirror 10, a stress transmission from the torsionbeam 30 is effectively blocked. Because such projecting parts of theconnecting ribs 22 are near the rotation axis (or rocking axis), inertiadoes not increase, which is advantageous for a high speed drive.

FIGS. 14A through 14C are views showing an example of a mirrordeformation amount and a stress distribution of the optical scanningdevice of the first embodiment. FIG. 14A is a view showing an example ofthe mirror deformation amount of the optical scanning device of thefirst embodiment. FIG. 14A proves that if the rib 21 is provided on theback side of the mirror supporting part 20 as the mirror deformationprevention structure, the deformation amount of the mirror 10 is almostzero, and the mirror 10 is sufficiently flat.

FIG. 14B is a view showing the stress distribution of the opticalscanning device of the first embodiment, including the projecting partof the connecting rib 22. FIG. 14B proves that by projecting the ribs 21(i.e., connecting ribs 22) toward the torsion beam 30 beyond the edge ofthe mirror 10, the stress concentrates on the projecting part, whichbecomes a stress relaxation part of the torsion beam 30.

FIG. 14C is a view showing a stress distribution in a mirror 10 plane.As shown in FIG. 14C, stress is only minimally generated in the mirror10 plane. This is because the stress from the torsion beam 30 isabsorbed in the projecting part of the connecting ribs 22, and is nottransferred to the mirror 10.

Second Example

FIGS. 15A through 15D are views showing a configuration and performanceresults of an optical scanning device of a second example. FIG. 15A is aview showing a cross-sectional configuration of a torsion beam 30 of theoptical scanning device of the second example. As shown in FIG. 15A, aslit 31 is provided in the center of a torsion beam 30, and torsionbeams 30L, 30R having a cross-sectional shape similar to a square onboth sides of the slit 31. The left torsion beam 30L and the righttorsion beam 30R have the same cross-sectional configuration, and thewidth is expressed as W, and the thickness is expressed as T.

FIG. 15B is a view showing a plane configuration on the back side of theoptical scanning device of the second example. As shown in FIG. 15B, theoptical scanning device of the second example has a configurationsimilar to the optical scanning device of the first embodiment describedin FIGS. 13A and 13B. Specifically, the optical scanning device of thesecond example includes connecting ribs 22 that project toward thetorsion beam 30 more than does the circumference of the mirror 10, and aprojecting amount of the connecting rib 22 from the mirror 10 isexpressed as X mm.

FIG. 15C is a view showing the relationship between a mirror flatness λin the maximum inclination and a nonlinearity coefficient β. In FIG.15C, W means a width of one side of a torsion beam; T means a torsionbeam thickness; W/T means a squareness ratio of one side of the torsionbeam; X means a rib projecting amount; λ means a mirror flatness in themaximum inclination; and β means a nonlinearity coefficient. Moreover,characteristics shown by solid lines express the mirror flatness λ, andcharacteristics shown by broken lines express the nonlinearitycoefficient β.

In the characteristics shown by the broken lines in FIG. 15C, as thesquareness W/T of the torsion beam is small and close to one (i.e.,close to a square), and as the projecting amount X mm decreases, thenonlinearity coefficient β decreases. However, values of thenonlinearity coefficient β change are relatively small, even though therib projecting amount X changes.

On the other hand, in the characteristics shown by the solid lines inFIG. 15C, the mirror flatness λ takes local minimum values aroundW/T=1.8, X=0.1 mm. At X=0.1 mm, the nonlinearity coefficient β does notexactly have the optimal values, but as mentioned above, because changesof the nonlinearity coefficient β are not so large and a configurationthat projects the ribs 21 aims at the mirror deformation prevention,W/T=1.8, X=0.1 mm are made the optimal values.

FIG. 15D is a view showing the relationship among the squareness W/T ofthe torsion beam 30, the mirror flatness λ in the maximum inclination,an axis beam maximum stress σ in the maximum inclination, and thenonlinearity coefficient β. In FIG. 15D, from the results of FIG. 15C,the projecting amount X is fixed at X=0.1 mm. Then, changes in themirror flatness λ in the maximum inclination, the axis beam maximumstress σ in the maximum inclination, and the nonlinearity coefficient βare measured, changing the squareness W/T of both sides 30R, 30L of thetorsion beam 30.

As shown in FIG. 15D, the nonlinearity coefficient β decreases as thesquareness W/T decreases and approaches one (i.e., as approaching asquare). The results can be said to be natural because bringing both ofthe sides 30R, 30L of the torsion beam 30 close to a square isfundamentally performed for the nonlinearity measures.

On the other hand, the mirror flatness λ in the maximum inclinationtakes the minimum value at W/T=1.76. In addition, though the axis beammaximum stress σ in the maximum inclination decreases as W/T increases,there is no problem as long as the maximum stress σ is not more than anallowable stress of the torsion beam 30. The axis beam maximum stressesσ in the maximum inclination shown in FIG. 15D are all values withoutany problems.

From FIG. 15C, an optimal range of the rib projecting amount X is0.05≦X≦0.15 mm, and X=0.1 mm is the optimal value.

Also, from the characteristics of the axis beam maximum stress a in themaximum inclination and the nonlinearity coefficient β shown in FIG.15D, the optimal range of W/T is 1.7≦W/T≦1.9, and the optimal value isW/T=1.76.

In this way, by adjusting the rib projecting amount X and W/T of bothsides 30R, 30L of the torsion beam 30, the maximum stress σ applied tothe torsion beam 30 is made a magnitude without a problem, and themirror flatness λ and the nonlinearity coefficient β can be reduced.

(Crosstalk Prevention Structure to Vertical Driving Beam in HorizontalDriving)

FIG. 16A through 16C are views to illustrate crosstalk generated ifthere is a movable frame 60 without a rib on the back side in theoptical scanning device of the first embodiment. FIG. 16A is aperspective view showing a configuration on the upper side of an opticalscanning device using a movable frame 60 without a rib on the back side.FIG. 16B is a view showing a configuration on the back side of theoptical scanning device using the movable frame 160 without the rib onthe back side. FIG. 16C is a view showing a horizontal driving state ofthe optical scanning device using the movable frame 160 without the ribon the back side.

As shown in FIGS. 16A and 16B, if the optical scanning device isconfigured by using the movable frame 160 without the rib, the movableframe 160 is configured as a beam with the same thickness as the otherbeams.

As shown in FIG. 16C, if the optical scanning device is configured byusing the movable frame 160 without the rib, the second drive beams 70that are vertical drive beams largely deform by horizontal driving bythe torsion beams 30. In other words, so-called crosstalk that affectsthe vertical driving when the horizontal driving occurs.

FIGS. 17A through 17C are views to illustrate crosstalk that isgenerated even if a movable frame 60 including a rib on the back side isused. FIG. 17A is a perspective view showing a configuration on theupper side of the optical scanning device using the movable frame 60with the rib on the back side. FIG. 17B is a perspective view showing aconfiguration on the back side of the optical scanning device using themovable frame 60 with the rib on the back side. FIG. 17C is a viewshowing a horizontal drive state of the optical scanning device usingthe movable frame 60 with the rib on the back side.

As shown in FIGS. 17A and 17B, by using the movable frame 60 including arib on the back side, the movable frame 60 has a degree of thickness,and is configured as a frame with high rigidity.

However, as shown in FIG. 17C, when the optical scanning device isdriven horizontally by using the first drive beams 50, the second drivebeams 70 that are a vertical drive beams still deform.

FIGS. 18A through 18C are views to illustrate a crosstalk preventionstructure to the vertical drive beams during the horizontal drive of theoptical scanning device of the first embodiment. FIG. 18A is aperspective view showing a configuration on the upper side of theoptical scanning device of the first embodiment. FIG. 18B is aperspective view showing a configuration on the back side of the opticalscanning device of the first embodiment. FIG. 18C is an enlarged viewshowing the crosstalk prevention structure of the optical scanningdevice of the first embodiment.

As shown in FIG. 18A, the crosstalk prevention structure is not providedon the upper side of the optical scanning device.

On the other hand, as shown in FIG. 18B, on the back side of the opticalscanning device of the first embodiment, a movable frame 60 including arib is provided, and plural crosstalk preventing ribs 81-83 are providedon a connection 80 between the second drive beam 70 and the movableframe 60. Here, in FIG. 18B, the second drive beams 70 include ribs 72in places other than the connection 80 with the movable frame. The ribs72 are for harmonic superposition prevention when the optical scanningdevice is driven in a vertical direction, and differ from the ribs forthe crosstalk prevention. For example, when the second drive beams 70are driven at 60 Hz, sometimes the harmonics of multiple numbers of 60Hz such as 120 Hz, 240 Hz, 360 Hz and the like are superimposed. Theribs 72 for harmonic superposition prevention are provided to preventsuch superposition of the harmonics.

As shown in FIG. 18C, the connection 80 between the movable frame 60 andthe second drive beam 70 includes plural crosstalk preventing ribs 81-83that extend in an axial direction of the horizontal drive and in adirection perpendicular to the axial direction. The crosstalk preventingrib 81 is a rib that extends continuously from the movable frame 60parallel to the torsion beams 30 and the second drive beams 70.Moreover, the crosstalk preventing rib 82 is provided extending in awidth direction of the second drive beams 70, symmetrically with theharmonic superposition preventing rib 72. The crosstalk preventing rib83 is provided extending continuously from the movable frame 60 parallelto the crosstalk rib 82. Furthermore, the crosstalk preventing ribs 81,83 are configured to form a triangular hollow 84 outside the movableframe 60.

In this manner, by providing the crosstalk preventing ribs 81-83 betweenthe edge of the second drive beam 70 that is the vertical drive beam andthe movable frame 60, a transmission of the oscillation in thehorizontal drive to the second drive beams 70 can be prevented. Inparticular, by forming a triangle with the rib 81 and rib 83, thetriangular hollow 84 can absorb stress generated by the horizontaldrive, and reducing the influence on the second drive beams 70 of thevertical drive beam is possible.

FIG. 19 is a view showing a stress distribution during the horizontaldriving of the optical scanning device of the first embodiment includingthe crosstalk preventing ribs 81-83. As shown in FIG. 19, stress acts onthe mirror 10 driving horizontally, but stress does not occur in thesecond drive beams 70 of the vertical drive beam. Thus, by providing thecrosstalk preventing ribs 81-83 between the vertical drive beam edge andthe movable frame 60, transmission of the swinging oscillation in thehorizontal resonant drive to the vertical drive beams can be blocked.

(Frequency Change Prevention Structure)

FIGS. 20A through 20D are views to illustrate a frequency change causedby driving an optical scanning device without a frequency changeprevention structure, though similar to the optical scanning device ofthe first embodiment. FIG. 20A is a view showing a plane configurationof the optical scanning device without the frequency change preventionstructure. In FIG. 20A, the optical scanning device without thefrequency change prevention structure has a shape in which resonantdrive beams 150 in a horizontal direction extend from an inside wall ofthe movable frame 60. The resonant drive beam 150 extends vertically ata length L from the inside wall.

FIG. 20B is a view showing a cross-sectional configuration of themovable frame 60 and the resonant drive beam 150 without the frequencychange prevention structure shown in FIG. 20A. As shown in FIG. 20B, themovable frame 60 is made up of a whole SOI substrate including a thicksilicon substrate, and the resonant drive beam 150 is made up of a thinsilicon substrate via a buried oxide film 61. In addition, the resonantdrive beam 150 includes a drive source composed of a thin film of apiezoelectric device on the surface. In this way, a part including a ribsuch as the movable frame 60 is made up of the whole SOI substratecomposed by laminating the thick silicon substrate on the back side, theoxide film, and the thin silicon substrate on the upper side. On theother hand, a part constructing a beam such as the resonant drive beam150 is made up of only the thin silicon substrate on the upper side. Inthis respect, the optical scanning device of the first embodiment issimilar.

FIG. 20C is a view showing a state of driving the resonant drive beam150. The drive source 151 repeats expansion and contraction depending onthe polarity of the drive voltage, by which the resonant drive beam 150oscillates up and down. At this time, because the buried oxide film 61sandwiched by the resonant drive beam 150 and the movable frame 60becomes a supporting point of the up and down drive, and the buriedoxide film 61 is a member like a glass with few elasticity, the buriedoxide film 61 has a high brittleness and is easily damaged. Accordingly,sometimes a crack 62 occurs by the up and down driving of the resonantdrive beam 150, and the oxide film 61 is damaged.

FIG. 20D is a graph showing an example of the relationship between anintegrated drive time of the resonant drive beam 150 and a resonantfrequency change rate. As shown in FIG. 20D, if the resonant drive beam150 is continuously driven, the crack 62 occurs at the supporting pointof the oxide film 61 at a certain time Tc, and an apparent length L ofthe resonant drive beam 150 increases to (L+α), which causes thefrequency to shift lower and to change.

FIGS. 21A through 21C are views showing a stress distribution of anoptical scanning device without a frequency change prevention structureduring the horizontal driving. FIG. 21A is a view showing a stressdistribution on the upper side of the optical scanning device withoutthe frequency change prevention structure in the horizontal driving.FIG. 21B is a view showing a stress distribution on the back side of theoptical scanning device without the frequency change preventionstructure in the horizontal driving. FIG. 21C is an enlarged viewshowing a stress distribution of a supporting point part of the drivebeam on the back side of the optical scanning device without thefrequency change prevention structure in the horizontal driving.

As shown in FIGS. 21A and 21B, the resonant drive beams 150 are coupledto the movable frame 60 in a state of extending vertically from themovable frame 60. Also, FIG. 21C proves that stress is in a state easilyoccurring in a root part 63 that becomes a supporting point of theresonant drive beams 150.

FIGS. 22A and 22B are views to illustrate a first frequency changeprevention structure of an optical scanning device of the firstembodiment. FIG. 22A is a view showing a plane configuration of anoptical scanning device of the present embodiment including a frequencychange prevention structure. In FIG. 22A, a root part 52 connecting withthe movable frame 60 of the first drive beam 50 is not connected to theinside wall of the movable frame 60 vertically; a curved shape part 53having a rounded structure short of the movable frame 60 is formed; andthe root part 52 is coupled to the movable frame 60 via the curved shapepart 53. In other words, a plane configuration of the first drive beam50 has the curved shape part 53 that is cut inward at a position nearthe movable frame 60 but not reaching the movable frame 60 in a sideconnecting the movable frame 60 with the coupling beam 40.

FIG. 22B is an enlarged view showing the root part 52 shown in FIG. 22A.In FIG. 22B, the curved shape part 53 that is cut inward is formed at adistance D from the supporting point 64 that is a border between themovable frame 60 and the first drive beam 50. Because the curved shapepart 53 has an effect of dispersing and relaxing stress, by forming thecurved shape part 53 more inward than the supporting point 64 of themovable frame 60, the stress that concentrates on a supporting point 64(see FIG. 23C) can be dispersed to the curved shape part 53. This makesit possible to protect a part of the oxide film 61 of the movable frame60, and to make the part of the oxide film 61 difficult to be damagedeven if driven continuously.

FIGS. 23A through 23C are views showing a stress distribution in thehorizontal drive of the optical scanning device of the first embodimentincluding the first frequency change prevention structure. FIG. 23A is aview showing the stress distribution on the upper side of the firstdrive beam 50 in the horizontal drive of the optical scanning device ofthe first embodiment. FIG. 23B is a view showing the stress distributionon the back side of the first drive beam 50 in the horizontal drive ofthe optical scanning device of the first embodiment. In FIGS. 23A and23B, the curved shape part 53 is formed in the root part 52 of the firstdrive beam 50.

FIG. 23C is an enlarged view showing the root part 52 of the first drivebeam 50 shown in FIG. 23B. In FIG. 23C, the curved shape part 53 isformed at a position more inside than the supporting point 64 and closerto the movable frame 60 than to the coupling beam 40. The stressdistribution occurs in an area more inside than the curved shape part53, and does not reach the part of the supporting point, as shown inFIG. 23C. Here, if a distance D between the supporting point 64 and theclosest position to the supporting point 64 of the curved shape part 53is so short that a generated stress cannot be separated from thesupporting point of the up and down driving, but so long as swingingsensitivity decreases, there may be caused a concern of not beingcapable of meeting a required specification. Therefore, the distance Dneeds to be set at a proper value, for example, which may be set at 0.1mm.

FIG. 24 is a view showing an integrated drive time and a resonantfrequency change rate of the optical scanning device of the firstembodiment. FIG. 24, different from the example of FIG. 20D, shows thateven though the integrated drive time becomes long, the resonantfrequency change rate is constant, and the resonant frequency is keptconstant.

Thus, according to the optical scanning device of the first embodiment,with respect to a planar shape of the first drive beam 50 that performsa resonant drive, by forming the curved shape part 53 cutting inward ata position near the movable frame 60 but not reaching the movable frame60, it is possible to prevent a stress concentration on the supportingpoint 64 of a border between the movable frame 60 and the first drivebeam 50, to prevent the oxide film 61 of the movable frame 60 from beingdamaged, and to keep a drive frequency constant.

FIGS. 25A through 25D are views to illustrate an optical scanning deviceof the first embodiment further including a second frequency changeprevention structure adding to the first frequency change preventionstructure. FIG. 25A is a plane configuration view on the upper side ofthe optical scanning device of the first embodiment including the firstand second frequency change prevention structure. FIG. 25B is a planeconfiguration view on the back side of the optical scanning device ofthe first embodiment including the first and second frequency changeprevention structure. In FIGS. 25A and 25B, a planar shape of the firstdrive beams 50 differs from the shape shown in FIGS. 22 and 23 in thatsides of the first drive beam 50 include not only the curved shape part53 but also a constricted part 54 that cuts toward the mirror 10. Inthis way, by providing not only the curved shape part 53 but also theconstricted part 54 that cuts inward, the stress of the first drive beam50 can be further moved inward and dispersed.

FIG. 25C is an enlarged plan view showing an inside of the movable frame60 of the optical scanning device of the first embodiment. FIG. 25D isan enlarged view showing a stress distribution of a side part of thefirst drive beam 50. As shown in FIG. 25C, with respect to the firstdrive beam 50, the curved shape part 53 and the constricted part 54continue and form the side part of the first drive beam 50. Also, asshown in FIG. 25D, by providing the constricted part 54 on the couplingbeam 40 side near the mirror 10, the stress is shifted toward theconstricted part 54, and the stress is remarkably reduced in the root ofthe movable frame 60 side. In other words, by forming the constrictedpart 54 distant from the supporting point 64 of the movable frame 60,the stress in the supporting point 64 can be widely moved to theconstricted part and can be distinctly reduced.

FIGS. 26A through 26C are views showing stress distributions of opticalscanning devices of respective embodiments in a comparative way. FIG.26A is a view showing a stress distribution of an optical scanningdevice without a frequency change prevention structure. FIG. 26B is aview showing a stress distribution of an optical scanning device withonly a first frequency change prevention structure. FIG. 26C is a viewshowing a stress distribution of an optical scanning device with thefirst and second frequency change prevention structures.

In FIG. 26A, a stress is applied to the supporting point 64 that is aroot of the resonant drive beam 150, and a stress distribution that maycause damage is shown.

On the other hand, in FIG. 26B, by providing the curved shape part 53more inward than the supporting point 64, a stress is generated moreinward than in the curved shape part 53, and the stress reaching thesupporting point 64 can be prevented.

Furthermore, in FIG. 26C, by providing the constricted part 54 on themirror 10 side, which is the coupling beam 40 side, stress is moved tothe constricted part 54, and the stress is hardly generated more to theexterior than the curved shape part 53.

FIG. 27 is a graph showing stress measurement results in the supportingpoint of the first drive beam 50 of the respective frequency changeprevention structures shown in FIGS. 26A through 26C. As shown in FIG.27, compared to the beam shape without the frequency change preventionstructure of FIG. 26A, by providing the first frequency changeprevention structure, the beam shape of FIG. 26B greatly decreases theroot stress. In addition, by further providing the second frequencychange prevention structure, the beam shape of FIG. 26C furtherdecreases the generated stress in the root than does the beam shape ofFIG. 26B.

In this manner, by providing the frequency change prevention structureof the curved shape part 53 and the constricted part 54 for the firstdrive beam 50, destruction of the oxide film 61 of the supporting point64 of the movable frame 60 can be prevented, and the optical scanningdevice can be driven by keeping the frequency constant even if drivencontinuously for a long time.

Second Embodiment

FIG. 28 is a view showing an example of an optical scanning device of asecond embodiment. In the optical scanning device of the secondembodiment, only a structure of a rib 21A provided on the back side ofthe mirror supporting part 20 differs from the optical scanning deviceof the first embodiment. Hence, with respect to the other components,the same numerals as the description hereinbefore are used and thedescriptions are omitted.

The rib 21A of the optical scanning device of the second embodimentincludes coupling ribs 22A, arc-like ribs 23A, chordal ribs 24A,longitudinal ribs 25A, and a transverse rib 26A, which have a similarstructure to those in the optical scanning device of the firstembodiment. The optical scanning device of the second embodiment differsfrom that of the first embodiment in that penetration ribs 27A extendedfrom the longitudinal rib 25A cross the chordal ribs 24A, and furtherreach the inside wall of the arc-like ribs 23A.

According to the optical scanning device of the second embodiment, byfurther providing the penetration ribs 27A that penetrate the chordalribs 24A and reach the arc-like ribs 23A, deformation of the mirror 10can be further reduced.

Third Embodiment

FIG. 29 is a view showing an example of an optical scanning device of athird embodiment. The optical scanning device of the third embodimentdiffers from that of the first embodiment in that a rib 21B includeslongitudinal ribs 25B and penetration ribs 27B that connect a point Awith a point D, and a point B with a point C respectively and form ashape crossing in an X-like shape. Since the other coupling ribs 22B,arc-like ribs 23B, chordal ribs 24B and transverse ribs 26B have aconfiguration similar to corresponding ribs of the optical scanningdevice of the second embodiment, the descriptions are omitted.

According to the optical scanning device of the third embodiment,implementing a mirror deformation prevention structure strong againstdiagonal stress is possible.

Fourth Embodiment

FIG. 30 is a view showing an example of an optical scanning device of afourth embodiment. The optical device of the fourth embodiment differsfrom those of the first and second embodiments in that a rib 21Cincludes connecting ribs 22C, arc-like ribs 23C and chordal ribs 24Cthat are formed as a single large mass. In this manner, constructing theconnecting ribs 22C in an integrated manner with the arc-like rib 23Cand the chordal rib 24C is possible. Because the mirror supporting part20 is reinforced more solidly, an effect of preventing the mirrordeformation can be certainly enhanced. Here, since the configuration ofthe longitudinal ribs 25C and the transverse ribs 26C is similar tothose of the first and second embodiments, the descriptions are omitted.

Fifth Embodiment

FIG. 31 is a view showing an example of an optical scanning device of afifth embodiment. The optical scanning device of the fifth embodimentdiffers from that of the third embodiment in that a rib 21D includesconnecting ribs 22D, arc-like ribs 23D and chordal ribs 24D that areformed as a single large mass. In this case also, because the mirrorsupporting part 20 is reinforced more solidly, an effect of preventingthe mirror deformation can be surely improved. Here, since theconfiguration of the longitudinal ribs 25D and the transverse ribs 26Dare similar to that of the third embodiment, the descriptions areomitted.

In this way, according to embodiments of the present invention, it ispossible to reduce generation of a nonlinear oscillation and to preventa mirror deformation.

The embodiments of the present invention can be applied to an imageprojection apparatus such as a projector that projects an image bydeflecting light.

Embodiment (1) is an optical scanning device including:

-   -   a mirror;    -   a mirror supporting part to support the mirror on an upper        surface; and    -   a pair of torsion beams to support the mirror supporting part        from both sides in an axis direction and to drive the mirror        supporting part so as to swing the mirror supporting part around        the axis by being twisted themselves,    -   wherein each of the torsion beams includes a slit approximately        parallel to the axis direction.

Embodiment (2) is the optical scanning device as described in Embodiment(1),

-   -   wherein the slit is a single slit and provided in a center of        the torsion beam.

Embodiment (3) is the optical scanning device as described in Embodiment(1),

-   -   wherein a rib is provided on a back side of a connection between        the mirror supporting part and the torsion beam.

Embodiment (4) is the optical scanning device as described in Embodiment(3),

-   -   wherein the rib projects toward the torsion beam more outward        than an edge of the mirror.

Embodiment (5) is the optical scanning device as described in Embodiment(3),

-   -   wherein an inner edge of the slit reaches more inward than an        outer edge of the rib, and a part of the rib is exposed from the        slit.

Embodiment (6) is the optical scanning device as described in Embodiment(3),

-   -   wherein the rib has arc-like walls along a circumference of the        mirror supporting part extending from the connection, and a        chordal wall connecting edges of the arc-like walls to each        other formed on the back side of the mirror supporting part.

Embodiment (7) is the optical scanning device as described in Embodiment(6),

-   -   wherein the rib is provided at each side of the mirror        supporting part, and further includes a longitudinal part        connecting the chordal wall of one side to the chordal wall of        the other side, extending parallel to the axis direction.

Embodiment (8) is the optical scanning device as described in Embodiment(3),

-   -   wherein the rib further includes a transverse part extending by        passing a center of the mirror.

Embodiment (9) is the optical scanning device as described in Embodiment(1), further including:

-   -   a movable frame surrounding the mirror, the mirror supporting        part and the pair of torsion beams;    -   a pair of first drive beams, the first drive beams having first        edges connected to and supported by inside walls of the movable        frame, being opposite to each other in a direction perpendicular        to the axis, and being configured to generate a driving force to        swing the mirror supporting part in a first direction by an up        and down driving; and    -   coupling beams to couple second edges of the first drive beams        with the torsion beams and to transmit the driving force to the        torsion beams,    -   wherein a planar shape of the first drive beam includes a curved        shape part cut inward in a side connecting the movable frame        with the coupling beam in a position near the movable frame.

Embodiment (10) is the optical scanning device as described inEmbodiment (9),

-   -   wherein the planar shape of the first drive beam includes a        constricted shape part cut inward most in a location near the        coupling beam in the side.

Embodiment (11) is the optical scanning apparatus as described inEmbodiment (9), further including

-   -   second drive beams coupling to the movable frame from outside        and configured to swing the mirror supporting part in a second        direction through the movable frame; and    -   a crosstalk preventing rib crossing in the first and the second        directions on a back side of a connection between the movable        frame and the second drive beams.

Embodiment (12) is the optical scanning device as described inEmbodiment (11),

-   -   wherein the movable frame includes an outer wall extending in a        direction different from the first and the second directions in        the connection, and    -   wherein the outer wall and the crosstalk preventing rib form a        triangular hollow.

Embodiment (13) is the optical scanning device as described inEmbodiment (9),

-   -   wherein the first drive beam generates a drive force by resonant        oscillation, and the second drive beam generates a drive force        by non-resonant oscillation.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority orinferiority of the invention. Although the embodiments of the presentinvention have been described in detail, it should be understood thatthe various changes, substitutions, and alterations could be made heretowithout departing from the spirit and scope of the invention.

What is claimed is:
 1. An optical scanning device, comprising: a mirror;a first drive beam configured to swing the mirror around a first axis;and a second drive beam configured to swing the mirror around a secondaxis; wherein the second drive beam is provided in such a manner that aplurality of beams extending in a direction intersecting with adirection of the second axis are joined with adjacent beams at edgeportions thereof, and thereby has a zigzag shape, and each of theplurality of beams includes a rib extending in a direction of a width ofthe beam and protruding from a surface of the beam.
 2. The opticalscanning device as claimed in claim 1, wherein the rib extends in adirection orthogonal to that of the first axis.
 3. The optical scanningdevice as claimed in claim 1, wherein the rib is formed on a straightline traversing the plurality of beams.
 4. The optical scanning deviceas claimed in claim 1, wherein a drive source is formed on a surface ofthe second drive beam in each rectangular unit including no curved lineportion and the rib is positioned on a back face of the second drivebeam and near a boundary between the rectangular unit and a curved lineportion.
 5. The optical scanning device as claimed in claim 1, furthercomprising a movable frame configured to support the mirror, wherein thesecond drive beam is provided to be opposed to and interpose the movableframe between both left and right sides thereof.
 6. The optical scanningdevice as claimed in claim 4, wherein the second drive beam isconfigured in such a manner that voltages with different polarities areapplied to adjacent drive sources in each rectangular unit.
 7. Theoptical scanning device as claimed in claim 1, wherein a driving forcefor the second drive beam is generated by a non-resonant oscillation. 8.The optical scanning device as claimed in claim 1, wherein the rib is arib for prevention of superimposition of a higher harmonic wave in acase where the mirror is driven around the second axis, so that a higherharmonic wave with a frequency being a multiple of a driving frequencyof the second drive beam is prevented from superimposing.